One of the things that famously feed the never-ending debate about tennis' greatest player is the sport's awkward division between the Open Era, which began in 1968, and what came before. We judge the greatness of our sport's champions almost exclusively by how many Grand Slam trophies they have, a fatally flawed system if you're interested in being fair. Before 1968, the best of the best played on the amateur tour -- which included the Grand Slam events -- only until they made their names. Then they jumped to the barnstorming pro tour so they could put some money in the bank. The great 1930s champion Ellsworth Vines ended his Grand Slam career at 22. Mid-century greats Pancho Gonzales and Ken Rosewall were in their very early twenties when they turned pro; they would be years past their primes before they were allowed to return to Wimbledon and Forest Hills. Then there's the fact that the reputation of the four Slams waxed and waned in the early years of the Open Era. Even as late as the 1980s, few players bothered to play all four legs of the Grand Slam every year.

So, as amazing as Federer's and Nadal's Grand Slam totals are (the 32-year-old Federer has the record with 17 singles titles, the 28-year-old Nadal is tied with Pete Sampras for second with 14), it's important to put those totals in context. Sidney Wood, a fine player in the 1930s who died at 97 in the middle of Federer's long reign as number one, and Jack Kramer, a champion in the '40s who also followed the game up until Federer's day, both maintained to the end of their lives that Don Budge was the greatest player of all time. Wood called the choice "a no-brainer." Offered Kramer: "Right away a lot of people are going to say I'm an old timer, pushing the guys of my era. Don't I know that the human body runs faster and jumps higher now than in the 1930s? And I say, yes, I know that, and will you please name me a better hitter than Ted Williams and a better singer than Caruso?" Wielding a "bludgeon backhand struck with a sixteen-ounce 'Paul Bunyan' bat," Budge was the first man to win the calendar-year Grand Slam, in 1938, but he won all of six major singles championships in total -- 11 fewer than the current standard. (Check out Wood's posthumously published memoir.)

In proclaiming Nadal the greatest ever, Agassi called this current period "the golden age of tennis," as we all have. But if it is, it surely isn't the first. Nadal has had to deal with Federer, Djokovic and Andy Murray. But Sampras had to deal with Agassi, Boris Becker, Stefan Edberg, Goran Ivanisevic, Jim Courier, Thomas Muster and Gustavo Kuerten. That looks like a legit golden age. Bjorn Borg had to deal with John McEnroe, Jimmy Connors, Guillermo Vilas, Roscoe Tanner, Vitas Gerulaitis, Adriano Panatta and Ilie Nastase. Pancho Gonzales had to deal with Jack Kramer, Pancho Segura, Ken Rosewall, Lew Hoad, Tony Trabert and Alex Olmedo. Having a bunch of truly great players all vying for big titles at the same time is nothing new.

There's another little problem. The Open Era/pre-Open Era divide isn't the only dramatic separation in the sport's history. There's a much more recent division that changed the sport almost as significantly.

The argument can be made that the ATP hasn't been kind to Federer. The Swiss great, who will begin his quest for a record eighth Wimbledon title later this month, grew up watching professional tennis matches played on low-bouncing grass courts and fast hard courts, when serve-and-volleyers such as Sampras, Becker and Edberg, Federer's idol and current coach, ruled the tour. That being the case, he learned a one-handed backhand and became accustomed to charging the net. By the time he reached his prime, however, his flashy, offensive-minded game had become an anachronism. In a vain attempt to broaden tennis' fan base, the tour slowed down court surfaces to increase the length of points. Wimbledon's grass and Roland Garros' clay remain different experiences, of course, but they're far more similar than they used to be. At the same time, new racket and string advances increased the power and spin of the typical groundstroke. Just like that, venturing to the net became the tennis equivalent of the Charge of the Light Brigade. In just a few short years, a style of play that had defined top-level tennis for half a century all but disappeared.

Is Nadal a great player? He certainly is. He can do it all, and he does do it all, on the biggest stages against the best competition on every court surface available. That said, he has had the good fortune of coming of age when the conditions -- slow courts, spin-maximizing technology -- perfectly fit his talents. You can say the same for Sampras during his time at Wimbledon and the U.S. Open, but of course that was not the case for him at the French and Australian championships.

You cannot say the same for Federer at any Grand Slam tournament. He has won a record number of major titles even though none of the tour's major court surfaces perfectly fit his talents. He played serve-and-volley tennis when, as a 19-year-old, he upset Sampras at Wimbledon in 2001. This was, arguably, Federer at his most natural and free flowing, a pure tennis spirit simply doing what came naturally. (Watch highlights of the match above.) By the time he won the tournament two years later, with the courts slower, he was spending most of his time in the backcourt. By all rights -- just look at the historical precedents, from Ellsworth Vines to John McEnroe -- his jazzy, improvisational game should fall flat on clay. McEnroe never won the French Open. Sampras reached the semifinals only once. Edberg pushed past the quarterfinals only one time, when he reached the final in 1989. Federer took the title in 2009 and has reached the final round four times, falling to the greatest clay-court player ever (Nadal) each time. He's won five clay-court Masters tournaments, two of them over Nadal.

This means Federer really is the best after all, Agassi be damned, right? Well ... maybe and maybe not. Who's to say Nadal couldn't have been successful as a serve-and-volley man at Wimbledon, if that had been necessary? Borg made the switch from backcourt spin demon to net-rushing lawn mower every year. He won back-to-back French Open and Wimbledon titles in 1978, '79 and '80, one of the greatest feats in the sport's history. Nadal is a good volleyer, and he does move closer to the baseline and hit flatter at SW19, where he's won twice. There's no reason to think he couldn't have taken his adjustments further. Plus, his French Open record is outrageous, mammoth, truly unbelievable. Okay, he frustrated Federer in Paris year after year, we get that: big lefty forehand to vulnerable one-handed backhand, rinse and repeat. But Djokovic was built from the ground up to beat Nadal on clay, and he's proven in Monte Carlo and Rome he can do it. But Nole is now 0-6 at the French Open against Rafa. There's something to be said for the intangibles, for the sheer will of a great champion. Nadal has it in spades.

And so the beat goes on. When looking at the big picture, choosing between Federer and Nadal is impossible. Maybe we should just follow Sidney Wood's and Jack Kramer's lead and agree that Don Budge was the greatest of all time.